Potassium thiosulfate (K.sub.2 S.sub.2 O.sub.3 -KTS) is a good liquid fertilizer and is used on soil, on foliage, or by injection through an irrigation system for commodity crops including corn and soybeans. At a 1:1 stoichiometric ratio, KTS has the highest K+S value of any liquid fertilizer.
Potassium thiosulfate is usually made by one of two processes. One common process involves contacting ammonium thiosulfate ((NH.sub.4).sub.2 S.sub.2 O.sub.3 -ATS) with potassium hydroxide (KOH) under elevated conditions. The other process uses sulfur dioxide (SO.sub.2) made from relatively inexpensive refinery off gases, contacts the gas with potassium hydroxide solution, and reacts the SO.sub.2 /KOH solution with sulfur.
Unfortunately, both of these processes are prone to a number of practical problems. One is that potassium hydroxide is a fairly expensive reagent whose market value fluctuates, often significantly (between about $200-$300/ton), in relation to the demand for chlorine. A second is the environmental risks associated with SO.sub.2 gases. The third problem with the conventional synthesis processes is the undesirable oxidation of sulfites that form sulfate solids that are not soluble in KTS and precipitate inside the pipes of the reactor system. The solids ultimately plug the lines and require extensive maintenance that offset the reduced cost of the sulfur source gases. Precipitated sulfate solids also require that the product be filtered before it can be packaged for sale to a customer. The filter and its maintenance represent additional undesired processing costs. A fourth drawback is the need to locate the KTS plant next to a refinery for capture of the refinery off gases.
It would be beneficial to have a method for making potassium thiosulfate that did not require the use potassium hydroxide as the potassium source.
It would be desirable to have a manufacturing process for potassium thiosulfate that did not use environmentally difficult SO.sub.2 gas, did not require filtration before packaging, and could be located in areas other than next to a petroleum refinery.
It would also be advantageous to have a manufacturing process that could operate at ambient to slightly elevated temperatures and thereby avoid the energy costs associated with heating systems to allow the reaction to proceed at a commercially viable rate.